In
investigating an outbreak, speed is essential, but getting the right
answer is essential, too. To satisfy both requirements, epidemiologists
approach investigations systematically, using the following 10 steps:

Prepare for field work

Establish the existence of an outbreak

Verify the diagnosis

Define and identify cases

Describe and orient the data in terms of time, place, and person

Develop hypotheses

Evaluate hypotheses

Refine hypotheses and carry out additional studies

Implement control and prevention measures

Communicate findings

The
steps are presented here in conceptual order. In practice, however,
several may be done at the same time, or they may be done in a different
order. For example, control measures should be implemented as soon as the
source and mode of transmission are known, which may be early or late in
any particular outbreak investigation.

Step 1: Prepare for Field Work

Before leaving for the field, you should:

Research
the disease and gather the supplies and equipment you will need

Make
necessary administrative and personal arrangements for such things as
travel, and

Consult
with all parties to determine your role in the investigation and who
your local contacts will be once you arrive on the scene.

Step 2: Establish the Existence of an Outbreak

One of your first tasks as a field investigator, or disease detective, is to
verify that a suspected outbreak is indeed a real outbreak. Some will turn
out to be true outbreaks with a common cause, some will be unrelated cases
of the same disease, and others will turn out to be unrelated cases of
similar but unrelated diseases. Before you can decide whether an outbreak
exists (i.e., whether the observed number of cases exceeds the expected
number), you must first determine the expected number of cases for the
area in the given time frame.

How, then, do you determine what is expected? Usually you can compare the current
number of cases with the number from the previous few weeks or months, or
from a comparable period during the previous few years. The sources of
these data vary:

For a
notifiable disease (one that, by law, must be reported), you can use
health department surveillance records.

For
other diseases and conditions, you can usually find data from local
sources such as hospital discharge records, death (mortality)
records, and cancer or birth defect registries.

If
local data are not available, you can make estimates using data from
neighboring states or national data, or you might consider
conducting a telephone survey of physicians to determine whether
they have seen more cases of the disease than usual. You could even
conduct a survey of people in the community to establish the
background level of disease.

Even if the current number of reported cases exceeds the expected number, the
excess may not necessarily indicate an outbreak. Reporting may rise
because of changes in local reporting procedures, changes in the case
definition, increased interest because of local or national awareness, or
improvements in diagnostic procedures. For example, if a new physician,
infection control nurse, or health care facility is reporting cases more
consistently than they were reported in the past, the numbers would go up
even though there might be no change in the actual occurrence of the
disease. Finally, particularly in areas with sudden changes in population
size, such as resort areas, college towns, and migrant farming areas,
changes in the number of reported cases may simply reflect changes in the
size of the population.

Whether or not you should investigate an apparent problem further is not strictly
tied to your verifying that an epidemic exists (that is, that the observed
number is greater than the number expected). As noted earlier, other
factors may come into play, including, for example, the severity of the
illness, the potential for spread, political considerations, public
relations, and the availability of resources.

Step 3: Verify the Diagnosis

In addition to verifying the existence of an outbreak early in the
investigation, you must also identify as accurately as possible the
specific nature of the disease. Your goals in verifying the diagnosis are
two-fold. First, you must ensure that the problem has been properly
diagnosedthat it really is what it has been reported to be. Second, for
outbreaks involving infectious or toxic-chemical agents, you must to be
certain that the increase in diagnosed cases is not the result of a
mistake in the laboratory.

Verifying the diagnosis requires that you review the clinical findings (the symptoms
and features of illness) and laboratory results for the people who are
affected. If you are at all uncertainty about the laboratory findings
(e.g., if they are inconsistent with the clinical findings), you should
have a laboratory technician review the techniques being used. If you
expect a need for specialized laboratory work (e.g., special culturing or
DNA analysis), you should begin obtaining the appropriate specimens,
isolates, and other laboratory material from a sufficient number of
patients as soon as possible.

Finally, you should visit several of the people who became ill. If you do not have
the clinical background to verify the diagnosis, a doctor or other
qualified clinician should do so. Regardless of your background, though,
you should see and talk to some of these people to gain a better
understanding of the disease and those affected by it. In addition, you
may be able to gather critical information by asking such questions as,
What were their exposures before becoming ill? What do they think caused
their illness? Do they know anyone else with the disease? Do they have
anything in common with others who have the disease? Conversations with
patients are very helpful in generating hypotheses about the cause,
source, and spread of disease.